1. Field of the Invention
This invention relates to the field of forming of glass sheets. The invention is particularly useful in the precision forming of a thin flat glass sheet to a section of a conical or cylindrical shape, while maintaining the optical characteristics of the flat glass sheet in the formed cylindrical or conical section. The invention may be applied to production of faceplates, windshields, goggles and various other items.
2. Description of the Prior Art
Optical applications such as motorcyclists' faceplates require the minimum possible distortion in the finished formed part. If distortion occurs across the field of view, the product is unacceptable for such applications. Moreover, as items made for such optical uses are often laminated to form safety glass, the component glass parts must be made both light in weight and formed to precise dimensions. Unfortunately, thin glass sheets are more susceptible to uneven deformation and distortion during forming than are thicker sheets. Therefore, prior art forming methods have not been completely successful in this area.
The prior art discloses various methods for forming glass sheets to the cylindrical and conical section. These methods may be categorized as two-part molding methods, or stamping processes, and one-part molding methods, or sagging processes. The present invention does not fall strictly into either of these categories. It is intended to employ the advantages and avoid the disadvantages of both processes, and thereby maintain the optical characteristics of the glass sheet while permitting forming to precise dimensions.
Two-part molding processes are those wherein glass sheets are heated until softened, then pressed between male and female molds conforming to the desired shape. A major disadvantage of such stamp molding is that the glass sheet must move relative to the extending portions of the forming member during the forming process. The point of contact between the glass and the forming member must change as the flat glass is brought into the desired curved shape. Resulting force along the plane of the glass sheet causes stretching and rippling of the sheet and degradation of optical characteristics due to variations in the curvature and thickness of the formed piece. In addition, such local variations in curvature make a part less easily laminated with other parts also having varying dimensions.
The present invention avoids exertion of force along the plane of the glass sheet being formed into a cylindrical or conical section, by use of forming wings that maintain contact with the glass sheet at the same point throughout the forming procedure. Thus the glass sheet is not stretched or compressed along its plane and the thickness of the sheet remains more uniform than could be obtained in a similar part made in a stamping process. Rather than forcing the sheet into a curving mold, the present invention bends the sheet around the curving form, by exerting force tangential to the arc followed by the point of contact between the forming member and the glass.
The prior art discloses one-part molding methods wherein the glass sheet is forced into a mold or around a form by a force other than that of another molding member. This force may be provided by gravity or by hot fluid or particle pressure. Such one-part molding methods, or sagging processes, are not subject to the above-described stretching problem, because the glass settles by its own weight into the mold or against the form. However, to produce such sagging, the glass must be heated well into the range of softening, and hence becomes very easily deformed. Variations in curvature of the formed part, and resulting degradations of optical quality also occur because the glass tends to flow in response to the gravity or other pressure exerted upon it.
The present invention allows forming of the glass as soon as the sheet reaches a temperature at which it can be deformed. The relatively low temperature of the sheet and the relatively short time needed to accomplish forming permit the part to be formed and rehardened before unwanted variations in curvature occur and thereby make the part unsuited for use in an optical application, such as an auto windshield or a motorcyclist's faceplate.
In the art of glass forming, heating glass sheets in molten salt baths is well known. The present invention takes advantage of the thermal inertia of such molten salt baths in its forming process. While prior art methods disclose molten salt baths for use at temperatures up to 1,000.degree. or 1,100.degree. F., the present invention is intended to operate at a temperature withn a few degrees of the softening temperature of the glass being used, 900.degree. to 1,000.degree. F. Rate of heat transfer is a function of temperature difference. Therefore, as the part being formed approaches the temperature of the bath, the rate of temperature change lessens, and precise temperature control of the glass can be thus accomplished by bath temperature control.